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The Effects of ACI on UHF Channel Allotments

Last month, this column introduced
a scheme for allocating
UHF band channels by analyzing
a set of contiguous channels for a given
community. For example Channels 30,
31, 32, 33, 34, and 35 might be allocated
to a given community (see Fig. 1).

However, each of these is subject
to adjacent channel interference (ACI).
Channels 30 and 35 would be exposed
to one undesired (U) signal, all others would be subject to
ACI from two adjacent channels. ACI is limited by the Desired
(D)-to-Undesired power ratios given in Table 5A and
FCC/OET Bulletin 69 (D/U = –26 dB).

If all seven transmitters on these contiguous channels
were co-sited, ACI would not occur because the power
received throughout the service area of these stations
should not vary by more than is allowed
in Table 5A. But what about communities
where there are multiple transmitter
sites? Allotments of contiguous channels
would probably require co-siting.

Our laboratory testing of both NTIA-approved
converter boxes and modern
DTV receivers shows that ACI is due to
the sideband splatter radiated in the D
channel by a DTV transmitter operating
on either channel adjacent to the D channel.
It is not due to third-order distortion
products generated in receivers.

REDUCING ACI THROUGH
RE-ALLOCATION
Ideally adjacent channels should not
be allocated to the same community. In
many cases there are three communities
far enough apart that adjacent channels can be allocated
to these communities without causing ACI. Consider three
such communities, A, B and C. Each community has different
channels allocated to it to avoid co-channel interference
(CCI). This may allow shorter spacing between A, B
and C.

This scheme eliminates CCI and ACI and thus would
meet the requirements set forth in the 2012 legislation
authorizing the FCC to auction UHF TV channels. However
many of the channels within each block may be what
were called taboo channels in the days of analog TV. These
would be subject to taboo channel interference (TCI).

According to the FCC’s Sixth Report and Order, there
would be no interference between DTV signals on taboo
channels because it was then believed that DTV receivers
could reject interference 60 dB greater than the D signal’s
received power. That is a D/U ratio of –60 dB, which was
highly improbable. The commission discovered that particular
error when testing set-top converter boxes for the
NTIA in 2008 and published in the December 2010 issue
of “IEEE Transactions on Broadcasting.” We found that modern
(2013) DTV receivers can reject taboo
signals as shown in Table 1.

Table 1: Threshold D/U ratios of modern (2013) DTV receivers

The limit D/U numbers for both ACI
(one U on channel N+1 only), and for taboo
interference— N+2 to N+6 inclusive—are
given for one U signal and for the worst
case combination of U signals on Channels
N+K and N+2K. Where the predicted D/U
is more negative than that shown in Fig.
1, reception may fail. In fact, at these D/U
numbers, 10 percent of our receivers failed.

Remember that the FCC D/U limit for ACI
is –26 dB. This is why we should minimize
adjacent channel allotments. In the FCC’s
lab tests of ATSC converter boxes for the
NTIA, two U ATSC signals on certain pairs
of channels of the form N+K and N+2K (K
in an integer), were tested. Interference was found and reported
in the above in the previously mentioned issue of
“IEEE Transactions on Broadcasting,” (pp. 441–451).

Later my colleagues and I repeated these tests on NTIA-approved
converter boxes and obtained similar results. In
subsequent papers published by the IEEE Consumer Electronics
Society, we reported that such interference affects
not only Channel N, but also a second Channel N+3K. This
doubles the threat of the heretofore unknown interference
mechanism.

My colleague, Linley Gumm and I recently tested
24 DTV receivers built in 2013. Modern ATSC receivers
have tuners that are implemented as an integrated
circuit. These tuners have quite different performance
characteristics concerning interference with respect to
the tuners in NTIA-approved converter boxes.

Those tuners were mostly single-conversion tuners
with the usual 44 MHz IF and were built with discrete
components. IC tuners are much smaller, consume less
power and cost less than tuners used before 2013 in ATSC
receivers. They are here to stay.

The 2013 vintage receivers that we tested are less robust
against interference from two or more U ATSC signals
than were the vintage 2008 NTIA-approved converter
boxes. This is unfortunate because we anticipate greater
density of DTV stations after the repacking than we now
have. That suggests more interference after repacking.

AVOID CONTIGUOUS
CHANNELS WITHIN A BLOCK
Fig. 2 shows the weighted third-order noise power for
both a five channel block (blue trace) and a seven channel
block (red trace). Note that there is much more noise
power per channel in the seven channel block than in
the five channel block. However there are more channels
for a given community with seven channels per block
than five.

Fig. 2: Third-order distortion products (noise) in each channel for a block of 5 or 7 Channels.

My colleague Stan Knight has analyzed the build-up of
third-order distortion products (noise) in the receiver’s
mixer in each channel of a block of six channels. Six channels
per block is a compromise between five and seven
channel blocks.

There is a difference of up to 7 dB in
noise levels between contiguous channels
and non-contiguous channels.

What these calculations show is that contiguous
channels within a block should be
avoided and that there is a significant difference
in noise in these channels depending
on the mix of channels in a given block.

The above data set we now know is not
the best possible, but that was not the objective
of these calculations. Knight found
that the noise levels differ substantially so it
would be necessary to find the optimum set
of channel numbers. This noise adds to the
receiver-generated noise to create the noise
floor under a given channel. This in turn may
determine the minimum usable received signal
power, hence the coverage of a station
where interference, not receiver noise, is the
limiting factor after repacking unless great
care is taken in the repacking algorithm.

As this noise (due to interference) is
generated in the receiver, the only way to
reduce it is to improve the linearity of future
tuners. With tuners-on-a-chip this may
be impossible because it would probably
require running more power in the tuner
circuitry which may cause the junction
temperature to exceed safe limits. Moreover,
improving linearity would only benefit
a small percentage of consumers while
any cost increases would be passed along
to all consumers. While future receivers
may manage this, what about the millions
of ATSC receivers now deployed?

After repacking, we will move from a
noise-limited coverage world to an interference-
limited world, but this situation is not
all bad. Remember that we have eliminated
ACI. But removing ACI and replacing it with
TCI is not progress. I suggest that we can
eliminate ACI as shown above and control
TCI by taking great care in the allotment
process. I suggest that we can do this under
the constraints imposed by the legislation
referred to above.

As the need for better spectrum efficiency
increases, some vast nationwide scheme
such as we have used in the past, may have
to be modified for the different regions of
the country. For example, the trios described
above are well-suited to the midwest, while
along both coasts there are long, high-population
density regions where two communities
need protection rather than three and
they could have more channels.

In summary, CCI can only be avoided by
the spacing between the transmitters and
directional (roof-mounted) receiving antennas.
ACI is completely avoided by not allocating
adjacent channels in the same community,
and TCI can be minimized by the choice
of channels allocated to a given community.

Happy Holidays to all my readers.

Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email
at cwr@bootit.com.